This invention relates to a method for casting molten metal articles, having varying thickness integral sections, within a combination permanent metal and temporary sand mold cavity. Portions of the casting cavity are formed by preselected portions of the permanent metal mold while other portions of the cavity are formed by a temporary sand liner within the cavity. This produces a generally equalized cooling effect on the molten metal, but with preselected areas that are rapidly chilled during the casting for providing harder portions in the cast article.
The underlying method for sand casting articles of varying thickness within a combined permanent mold and sand mold is described in U.S. Pat. No. 4,674,553 issued June 23, 1987 to Raymond H. Witt. That patent discloses the use of a permanent metal mold which is formed with an oversized mold cavity. The cavity is lined with sand to provide an interior sand surfaced casting cavity within which molten metal is poured and solidified.
The thickness of the sand lining is varied and is reversely coordinated, with the thickness of the corresponding areas of the cast article. That is, the sand liner is thinner where the article is thicker and the sand liner is thicker where the article is thinner. The respective thicknesses are correlated to produce a generally equalized cooling of the article, notwithstanding its varying wall thicknesses.
The foregoing method permits more rapid casting of metal articles from molten metal, that is, it permits opening the mold and removing the cast part more rapidly than otherwise expected.
Some cast articles, regardless as to whether formed in a permanent mold or a sand mold or a permanent mold lined with sand as described in U.S. Pat. No. 4,674,553, mentioned above, require localized hardened surface areas for their particular function. For that purpose, heat treating or other surface treatments are utilized for increasing the surface hardness. This requires additional treatment following casting, and, depending upon the configuration of the article, may provide a greater area of surface hardening than is necessary for the particular purpose of the article.
Thus, this invention is concerned with improving the casting of varying wall thickness articles by providing a means for hardening preselected areas of the article during the casting process.